1. Field of the Invention
The present invention relates to a suspension apparatus of a vehicle adapted to change suspension characteristics.
2. Description of Related Art
Recently, as a suspension apparatus of a vehicle is called an active suspension, it is proposed that the suspension apparatus is of a type in which its suspension characteristics can be changed in an arbitrary manner. The so-called active suspension is basically constructed such that a cylinder unit is interposed between an unsprung weight and a sprang weight and that the suspension characteristics are controlled by controlling a supply and discharge of operating liquid to and from the cylinder unit. Japanese Patent Publication (Kokoku) No. 14,365/1984 discloses a suspension apparatus which uses a cylinder unit as an actuator for adjusting a ride height of the vehicle body and which controls a posture of the body by controlling a pressure of the operating liquid in the cylinder unit.
In the so-called active suspension, suspension characteristics can largely be changed by supplying the operating liquid or discharging it to or from outside in order to carry out various control including control of the ride height of the vehicle body, control of a roll component of vibration of the body, control of a pitch component of vibration thereof, and so on.
In the active suspension, there has generally been used a ride height sensor for sensing the ride height of the vehicle body for control of the posture of the vehicle body so that the suspension control cannot be implemanted to a sufficient extent if the ride height sensor would be in trouble. Japanese Patent Unexamined Publication (kokai) No 289,417/1987 proposes that whether the ride height sensor is in trouble or not is judged by observing a velocity of changes in output values from the ride height sensor. Japanese Patent Unexamined Publication (kokai) No. 282,110/1986 proposes that a portion of plural ride height sensors is judged to be in trouble when output from the portion of the plural ride height sensors does not vary although outputs from the plural ride height sensors are actually varying.
It is thus to be noted that the active control of the suspension is impaired when a supply/discharge control valve for supplying or discharging the operating liquid to or from the cylinder unit is out of order, particularly when the supply/discharge control valve is fixed in such a state that the operating liquid is being supplied.